1. Field of the Invention
The invention relates generally to the field of radars. More particularly, the invention relates to radar receive antenna arrays. Specifically, a preferred implementation of the invention relates to an optimized design for radar receive antenna arrays.
2. Discussion of the Related Art
MF (medium frequency)/HF (high frequency)NIHF (very high frequency) radars and radios remain in demand even with the advent of satellites that extend the range of communication to global scales. HF signals have the advantage of spanning distances beyond the horizon limits of UHF and higher-frequency signals. Two modes account for this. Reflection of HF signals from the ionosphere is called the skywave mode, and permits radio signals to traverse half way around the world. In the surface-wave mode, the spherical earth diffracts vertically polarized signals beyond the visible horizon—even in the absence of an atmosphere or ionosphere. For surface wave propagation, the sea is better than land because of its higher conductivity. The drawback of MFIHFIVHF systems is the large antenna size needed for high gain and/or efficiency. High gain is achieved with conventional antenna design by requiring aperture sizes many wavelengths. For example, when ships are to be detected beyond the horizon by an HF surface wave radar, best results are obtained if a receive antenna can maximize the echo energy captured from a bearing point on the sea surface, thereby realizing a high directive gain with a narrow beam focused along that bearing. At 5 MHz, for example, the wavelength is 60 meters. To achieve 20 dB directive gain, a linear monopole array has at least 32 elements spaced 30 meters apart, spanning one kilometer and achieving an azimuth beamwidth of about 4° (although the elevation beamwidth is 45°). If element amplitude tapering is employed to reduce sidelobcs that may be unwanted, then even more elements over a longer aperture are needed to achieve the same directive gain. Mathematical solutions have shown that one can form very narrow beamed patterns with closely spaced array elements by employing phasings that nearly cancel the incoming signals after beamforming. This concept is known as superdirective gain—or often just supergain, which is a type of directive gain. If the pattern beamwidth is small and sidelobe levels are low, then its directive gain is large. However, the efficiency of these arrays is low. That is, the summed output signal after applying the element phasings is much smaller than the signal into each element. Thus, while the directive gain is high, the power gain is low. This has been the primary source of criticism and lack of acceptance of superdirective arrays in practice. Another case occurs where ncar-cancellation is desirable in HF surfacc-wave radars because overhead ionospheric layers occasionally reflect—like a mirror—the signal back to the radar. This occurs at ranges from 100 to 350 km, depending on the layer height. These very intense echoes destroy the ability to see surface targets at the same ranges as the layer heights. Although many receive antenna systems employ vertical dipole or monopole antennas that theoretically have an overhead null, in practice this is not nearly enough to eliminate all vestiges of the intense overhead echo. Another undesirable feature of conventional phased array antennas is the variation and degradation of pattern characteristics as one steers the beam to different bearings. In linear phased arrays, for example, steering more than 45° from the optimal broadside direction results in unacceptable main-beam broadening and increase in sidelobe levels. As shown in U.S. Pat. No. 5,361,072, incorporated herein by reference in its entirety, single antenna elements can be made more compact and less costly at the expense of efficiency. A design criterion was developed and revealed to guide the size reduction so that external noise always dominated internal noise. Optimal signal-to-noise ratio (SNR) was thereby always ensured. However, such techniques were applied to single antenna elements only, and an implementation of the theory has not been realized for antenna arrays. Accordingly, the requirements of a compact antenna array that possesses the signal-to-noise ratio of the arrays of the prior art (or similar thereto) have not been fully met. What is needed is a solution that simultaneously addresses these requirements.